Split redundant trunk architecture using passive splitters and path switching

ABSTRACT

A collapsed ring fiber optic system includes a service path and a protection path provides at a shallow water portion of the fiber optic system, to deal with any fiber cuts that may occur at the shallow water portion without loss of main trunk bandwidth. The service and protection paths meet at a branch point, which is preferably located at a deep water portion of the fiber optic system. A passive combiner or a 1×2 switch is provided at the branch unit, along with a detector and a processor, to determine whether any signals are being received from the service path, and if not, to reconfigure the system to accept signals from the protection path. At another shallow water portion of the fiber optic system, nearby where a destination is located, the signal provided on the optical path over the deep water portion is split into a service path and a protection path, to provide redundancy to deal with any fiber cuts that may occur. The fiber optic system may also be utilized for a land-based system, having high probability of fiber cut regions and low probability of fiber cut regions.

CORRESPONDING RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/850,141, filed May 8, 2001, now U.S. Pat. No. 6,556,319,which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention relates generally to an optical network architecture, andmore specifically to a split redundant trunk architecture that usespassive splitters and path switching, which provides for fiber cutprotection and equipment failure protection.

B. Description of the Related Art

For underwater optical networks, a problem exists in shallow waters dueto dragging boat anchors and the like, which may make contact with fiberoptic lines and thereby cause damage or cuts to those lines. Thisproblem also may occur for land-laid optical networks, whereby certainportions of fiber optic cable laid below ground are more susceptible todamage than other portions of the fiber optic cable. For example, if afiber optic cable is provided between Baltimore, Md. and New York, N.Y.,then there is a higher probability of damage to the fiber optic cablelocated at the two cities, due to building and road construction andrepair, than along locations between the cities in which the fiber opticcable is laid.

Presently, fiber optic systems use one of two schemes that incorporatepath diversity in regions where there is a high probability of fibercut. In one scheme, fiber bundle legs are split at branch units and halfof the fibers are routed along two different paths. In the other scheme,each wavelength division multiplexed (WDM) fiber is split/combined atthe branch units by wavelength using wavelength splitters and combiners.In either case, half of the bandwidth is routed over two separatediverse paths. If one of the two fiber bundles is cut in the regionwhere there is a high probability of fiber cuts, half of the totalbandwidth is lost in the region where there is a low probability offiber cuts. Accordingly, there is a need for a fiber optic system usinga branch unit to route entire fiber bundles diversely, to avoid losinghalf of the bandwidth when one or more of the fiber bundles is damagedin the region where there is a high probability of fiber cuts.

Typically, conventional optical communication systems comprise areceiving node and a transmitting node (Baltimore, Md. and New York,N.Y. in the aforementioned example) connected via optical fiber. Eachnode contains equipment for communication via optical fiber. Suchequipment includes channel equipment and WDM equipment. A fiber-baycomprises channel equipment and WDM equipment. Channel equipment isequipment that transmits and receives via a specific channel. A lineunit is a repeater that optically amplifies WDM signals on an opticalfiber.

SUMMARY OF THE INVENTION

The present invention is directed to an optical network architecturethat operates effectively when fiber cuts occur on service lines. Theoptical network architecture includes a primary branch path and asecondary branch path, wherein both paths are provided on a region ofhigh fiber cut probability of the optical network architecture, andwherein identical transmission signals are provided on the primary andsecondary branch paths. The primary and secondary branch paths meet at abranch point, wherein a branch unit is located at the branch point. Thebranch unit includes a combiner that combines signals received on theprimary and secondary branch paths, and outputs the combined signal ontoa main optical path. The main optical path is located at a lowprobability of fiber cut of the optical ring architecture. Optionally,multiple branches may be incorporated and combiners used on subsets offibers at each branch. The main optical path may branch multiple times,or a branched optical path may branch again for example.

In a first operation mode, at least one of the line units on thesecondary branch path (preferably the last one or last few line units onthat path that are closest to the branch unit) has its pump laser set toa zero or nearly-zero power output state, so as to attenuate any signalssent over the secondary branch path. In the first operation mode, eachof the line units on the primary branch path has its respective pumplaser set to a normal power output state. Alternatively, it can be apower output state anywhere between the zero (or near-zero) power outputstate and the maximum power output state (and it may even be the maximumpower output state in some circumstances).

At the output of the combiner there is a 2% tap with light provided to adetector, such as a photodiode detector. If the photodiode detector doesnot detect any signal or if the signal quality is poor at the output ofthe combiner for at least a fixed time period, then it is determinedthat the primary branch path has a problem, and then the at least oneline unit on the secondary branch path is instructed to set its pumplaser to the normal power output state, so that the backup signal willbe received by the combiner from the secondary branch path, due to theproblem in receiving the primary (also called “service”) signal from theprimary branch path. The line units on the primary branch pathoptionally are instructed to set their respective pump lasers to thezero power output state. After the primary branch path has been fixed,then the system can be set back to a first operating mode, in which thecombiner receives the primary signals from the primary branch path andnot the backup signals from the secondary branch path.

In an alternative configuration, a 1×2 switch (typically a highreliability switch) is provided at the branch unit instead of thepassive combiner, whereby signals are provided to the two inputs of the1×2 switch from both the primary branch path and the secondary branchpath. The primary branch input is provided to the output of the 1×2switch under normal operating conditions. When the output of the 1×2switch is detected to be below a threshold level, thereby indicating aproblem on the primary branch path, the 1×2 switch is switched toprovide the input from the secondary branch path to the output of the1×2 switch. The output of the 1×2 switch is provided to a main opticalpath, which provides fiber optic signals over a region having a lowprobability of fiber cuts.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will becomeapparent upon reference to the following detailed description and theaccompanying drawings, of which:

FIG. 1A is a block diagram of a fiber optic system in which a branchunit according to the invention may be used, in which a 2:1 passivecombiner is provided in the branch unit and a 50/50 passive splitter isprovided in the branch unit in a fiber optic architecture;

FIG. 1B is a block diagram of a fiber optic system in which a branchunit according to the invention may be used, in which a split redundanttrunk architecture is diagramed in greater detail;

FIG. 2 is a block diagram of a branch unit according to a firstembodiment of the invention;

FIG. 3 is a diagram of a split redundant trunk in a multi-node ringconfiguration, according to any of the embodiments of the invention;

FIG. 4 is a diagram of a point-to-point split redundant trunkconfiguration, according to any of the embodiments of the invention; and

FIG. 5 is a block diagram of a branch unit according to a secondembodiment of the invention, which includes a 1×2 switch instead of apassive combiner.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention includes a service transmission optical line and aprotection transmission optical line. Referring to FIG. 1A, which showsthe basic architecture of a fiber cut protection system in which abranch unit according to the invention may be utilized. A servicetransmission optical line is coupled to a first line terminatingequipment 105, and a protection transmission optical line is coupled toa second line terminating equipment 110. The same data is provided onboth the service transmission optical line and the protectiontransmission optical line. The first and second line terminatingequipment can be provided at a first location or connected by opticalfiber of the fiber optic system.

The service transmission optical line is provided on a first opticalbranch (also called first branch path), and the protection transmissionoptical line is provided on a second optical branch (also called secondbranch path). The signals received from the first and second opticalbranches are combined at a combiner 210. The combiner 210 outputs acombined signal onto a main optical path. At least one line unit 108A isprovided on the first branch path between the first line terminatingequipment 105 and the combiner 210, and at least one line unit 108B isprovided on the second branch path between the second line terminatingequipment 110 and the combiner 210. The first and second branch pathsare preferably provided in regions where there is a high probability offiber cuts (e.g., shallow water regions or urban land regions) in whichthe fiber optic system is laid.

Each of the line units 108A, 108B on the first and second branch pathsrespectively, as well as on the main optical path, has at least one pumplaser, which can be set to a power level from zero to a maximum value.The line units operate as repeaters for receiving an optical signal andfor outputting an optical signal that has the same information contentas the received optical signal, but with increased signal strength toaccount for any signal attenuation between adjacent line units. Undernormal operating conditions, at least one of the line units 108B on thesecond branch path has its respective pump laser set to a zero ornear-zero power output state, and all of the line units 108A on thefirst branch path have their respective pump lasers set to a normalpower output state. Under normal operating conditions, the last few lineunits, such as the last one to four line units on the second branch paththat are directly upstream of the branch unit, which itself may operateas a line unit, preferably have their respective pump lasers set to thezero or near-zero power output state to reduce spontaneous noise andprevent the secondary signal from interfering with the primary signal.

FIG. 1B shows more details of the fiber optic system. In FIG. 1B, thefiber-bays may comprise a variety of devices to accommodate customerinterface and signal transmission. Fiber-bay 120 is shown with a 50/50splitter 150 and a 2×1 switch 155. Similarly, fiber-bay 110 is shownwith a 50/50 splitter 160 and a 2×1 switch 165. As customer datato-be-transmitted enters fiber-bay 120, the 50/50 splitter 150 sendsdata to fiber-bay 115 to be sent down the service transmit path. Thesplitter 150 also sends data down the protection transmit path.Fiber-bay 110 then uses the 2×1 switch 165 to select the service path orthe protection path for data received at fiber-bay 110.

Similarly, customer data to-be-transmitted that enters fiber-bay 110 issplit by the 50/50 splitter 160 and sent to fiber-bay 105 to be sentdown the service transmit path. The splitter 160 also sends data downthe protection transmit path. Fiber-bay 120 then uses 2×1 switch 155 toselect the service path or the protection path for data received at thefiber-bay 120. In this configuration the customer does not need toactively select between the service and protection paths due to thebranch units or terminal switch performing that function.

In more detail, for a branch unit according to the first embodimentwhich utilizes a passive combiner, when the service path is notoperating normally, as detected by a detector provided at the output ofthe combiner (see FIG. 2, for example), then the last few pumpamplifiers of the line units in the protection path are set to provide anormal power output (somewhere between minimum and maximum power outputcapability), to provide the backup signal to the combiner to make up forthe system problem in the service path.

The first and second branch paths meet each other at a branch unit toform the beginning portion of the main optical path. Referring to FIG.2, which shows a first embodiment of the invention, the branch unit 200includes a combiner 210, which is preferably a 2×1 passive combiner. Thecombiner combines the signals received on the first and second opticalbranch paths and outputs the combined signal onto the main optical path.

A detector, which is shown as a photodiode 220, is provided at theoutput of the combiner 210. The photodiode 220 receives a signalcorresponding to 2% of the output of the combiner 210, by way of a lighttap 230 (e.g., splitter) placed at the output of the combiner 210. Ofcourse, other tap amounts may be utilized while remaining within thescope of the invention, such as a 1% to 5% tap. When the output signalis below a threshold level, as determined by a processor 240 thatreceives information supplied to it from the photodiode 220, it isdetermined that there is a problem on the first branch path. Thecommands to switch between service and protection paths may also comefrom the end nodes (if the problem exists at the end nodes), althoughthere will be propagation delays in that instance. This problem may bethat there is no signal, thereby signifying a fiber cut (or a problem atthe end nodes). In that case, all of the line units 108B on the secondbranch path are instructed to set the power level of their respectivepump lasers to a normal power output state, so that the combiner 210receives the backup signals output from the second branch path.

In a normal operation mode, the signals on the second branch path arenormally attenuated on the second branch path, and thus are either notprovided to the combiner 210 or are provided to the combiner 210 at avery low signal strength. However, when a problem with the first branchpath is detected due to a low level signal or no signal detected at theoutput of the combiner 210, then the signals from the second branch pathare provided to the combiner 210 at an increased power level byincreasing the power level of the last one to four line units 108B onthe second branch path. The power level of these line units is increasedfrom a zero or non-zero power output state to a normal power outputstate. At the same time, one or more line units 108A on the first branchpath may have their laser units set to output a zero or near-zero poweroutput. This is done to ensure that any noise received on the corruptedfirst branch path does not corrupt the reception of the protectionsignal (on the second branch path) by the combiner 210.

Once the problem on the first branch path has been corrected, then thesystem can return back to its normal operating condition. The problem onthe first branch path may be, for example, a cut on the fiber opticalline somewhere on the first branch path. This cut may have been causedby an anchor or fishing trawler causing damage to a fiber optic cablethat is placed at a shallow water region of a body of water.Alternatively, for fiber optic cable laid on land, the damage may be dueto ground digging that inadvertently cuts a fiber optic cable at anurban construction site. During the time when the first branch path isbeing repaired, the end of the first branch path provided to the branchunit is preferably coupled to a high voltage switch (not shown) at thebranch unit, so as to short that path to ground. This providesprotection for workers who are repairing the first branch path. Afterthe repair is complete, the first branch path is decoupled from the highvoltage switch.

The signal output by the combiner 210 on the main optical path travelsalong the entire distance of the main optical path from a first regionwhere there is a high probability of fiber cuts (e.g., Lisbon harbor orBaltimore City) to a second region where there is a high probability offiber cuts (e.g., New York harbor or New York City). The main opticalpath is laid on a third region where there is a low probability of fibercuts, such as a deep water region (e.g., Atlantic Ocean or PacificOcean, or a rural land region).

At the far end of the main optical path, the signal from the firstoptical path is split, by way of a splitter, onto a third branch pathand a fourth branch path. The signals on the third and fourth branchpaths are equal to each other and are 3 dB less in signal strength thanthe signal on the first optical path. The splitter is preferably a 50/50passive splitter (3 dB loss), and preferably has minimal wavelengthdependence (a flattening filter may be utilized with the splitter if ithas some degree of wavelength dependence). FIG. 1A shows a splitter 125that provides an identical signal to a third line terminal equipment 115and to a fourth line terminating equipment 120. The splitter 125duplicates the optical fiber such that a cut in either of the duplicatedfibers does not result in a loss of half the bandwidth of the splitfiber, as opposed to a conventional system that splices the fiber inhalf, such that a cut in either of the spliced fibers results in a lossof half of the bandwidth of the split fiber.

There is provided at least one line unit on the third branch path,between the splitter 125 and the third line terminating equipment 115,and there is also provided at least one line unit on the fourth branchpath, between the splitter 125 and the fourth line terminating equipment120. The third line terminating equipment 115 and the fourth lineterminating equipment 120 are provided at a second location of the fiberoptic system. In a normal mode of operation, signals received by thethird line terminating equipment 115 are utilized at the receive end,and the signals received by the fourth line terminating equipment 120can be passed to the third line termination equipment 115. When thethird line terminating equipment 115 determines that there is a failureon the third branch path, such as a fiber cut, the system is switched soas to utilize the signals received by the fourth line terminatingequipment 120 on the fourth branch path. The third and fourth linetermination equipment 115, 120 can be joined by WDM fibers andrepeaters, such as in a standard 1+1 or unidirectional path switchedring architecture (UPSR).

A similar protection path and service path exists for signals travelingin the opposite direction from the second location to the first locationof the fiber optical system. FIG. 1A shows the paths in both directions,between the first and second line terminating equipment 105, 110 at thefirst location, and the third and fourth line terminating equipment 115,120 at the second location.

The basic architecture of a split redundant trunk structure 100 in whicha branch unit (see FIG. 2, for example) according to the presentinvention may be utilized is shown in FIG. 1A and FIG. 1B. Thearchitecture of FIG. 1A and FIG. 1B can be implemented for either amulti-node ring configuration or a point-to-point configuration, asshown in FIGS. 3 and 4, respectively. This architecture is suitable forsubmarine networks where the collapsed portion in the center of thearchitecture is in the deep ocean where it is extremely rare to have afiber cut in that region. Alternatively, this architecture is suitablefor land-based networks where part of the network is laid out in urbanregions (in which much building and road construction typically takesplace) and where other parts of the network are laid out in rural orsuburban regions (in which less building and road construction typicallytakes place). Regardless of the application, a customer interface tothis architecture will generally not distinguish between a standardarchitecture and a split redundant trunk structure according to thepresent invention.

In the multi-node ring configuration 300 as shown in FIG. 3, first andsecond customer interface equipment (CIE) 311, 321 are coupled to afirst optical cross-connect unit (OXC) 330 at a first location. Thefirst OXC 330 is coupled to a first set of fiber-bays 335 and a secondset of fiber-bays 340. All connections to the first OXC 330 arepreferably OC-192c SR1 fiber optic connections. The first set offiber-bays 335 is coupled to a first optical branch path 302, and thesecond set of fiber-bays 340 is coupled to a third set of fiber-bays 345via control fiber optic lines. Signals from fiber-bays to fiber-bays aretypically WDM.

There are also a fourth set of fiber-bays 350, which are coupled to asecond optical branch path 304. Both the third set of fiber-bays 345 andthe fourth set of fiber-bays 350 are coupled to a second OXC 355,whereby the second OXC 355 is coupled to third and fourth CIEs 360, 365.The first and second optical branch paths 302, 304 are preferably 8transmit/8 receive wavelength division multiplexed (WDM) optical fibers.

The first and second optical branch paths 302, 304 are coupled to a mainoptical path 368 via a first branch unit 310, which includes splittersand couplers (not shown in FIG. 3). The splitters split signals receivedfrom a main optical path 368, and provide the split signals onrespective service paths and protection paths of the first and secondoptical branch paths 302 and 304. The couplers couple signals from therespective service and protection paths, to be sent out over the mainoptical path 368. Like the first embodiment, the first and secondoptical branch paths 302, 304 are provided at a first shallow waterregion (or a first land region where there is a high probability of afiber cut) in which the multi-node ring configuration is disposed.

The main optical path 368 travels along a deep water region (or a landregion where there is a low probability of a fiber cut), such as anocean floor (or a rural land area), and makes its way to a secondshallow water region (or a second land region where there is a highprobability of a fiber cut) at which a second branch unit 320 couplesthe main optical path 368 to third and fourth optical branch paths 306,308. The second branch unit 320 includes splitters and couplers (notshown in FIG. 3). The splitters split a signal sent from one of thefirst through fourth CIEs onto a service path and a protection path. Thecouplers couple signals received on the third and fourth optical branchpaths 306 and 308, which are destined for one or more of the firstthrough fourth CIEs, to provide the coupled signal onto the main opticalpath 368.

A fifth set of fiber-bays 370 are coupled to the third optical branchpath 306, and a sixth set of fiber-bays 372 are coupled to a seventh setof fiber-bays 374. The coupling of the sixth and seventh sets offiber-bays may be via fiber optic control lines. The fifth and sixthsets of fiber-bays 370, 372 are coupled to a third OXC 376. The thirdOXC 376 is coupled to fifth and sixth CIEs 378, 380.

There is also provided an eighth set of fiber-bays 382, which is coupledto the fourth optical branch path 308. The seventh and eighth sets offiber-bays 374, 382 are coupled to a fourth OXC 384. The fourth OXC 384is coupled to seventh and eighth CIEs 386, 388.

In the point-to-point collapsed ring architecture 400 shown in FIG. 4, afirst landing station 410 is shown, which includes first and second CIEs402, 404, a first OXC 405 coupled to the first and second CIEs 402 and404 and to first and second sets of fiber-bays 408 and 409. The firstset of fiber-bays 408 is coupled to a first optical branch path 411, andthe second set of fiber-bays 409 is coupled to a second optical branchpath 412. The first optical branch path 411 includes 8/8 WDM fibers,which route service signals, and the second optical branch path 412includes 8/8 WDM fibers, which route protection (or backup) signals. The8/8 WDM can take on other arrangements such as 6/6, 4/4, 2/2 and otherarrangements as would be readily apparent to one skilled in the art.

The first and second optical branch paths 411, 412 meet up with eachother at a first branch unit 415, which includes splitters and couplers(not shown in FIG. 4), to couple the branch paths to a main optical path414. The main optical path 414 is a fiber optical path that is locatedat a deep water region or rural land region, for which redundancy is notneeded due to a small likelihood of fiber cuts occurring in theseregions.

Also shown in FIG. 4 is a second landing station 425, which includesthird and fourth CIEs 432, 434, a second OXC 437 coupled to the thirdand fourth CIEs 432, 434 and to third and fourth sets of fiber-bays 436,438. The third set of fiber-bays 436 is coupled to a third opticalbranch path 442, and the fourth set of fiber-bays 438 is coupled to afourth optical branch path 444. The third optical branch path 442preferably includes 8/8 WDM fibers, which route service signals, and thefourth optical branch 444 path preferably includes 8/8 WDM fibers, whichroute protection or backup signals. The third and fourth optical branchpaths 442, 444 are coupled to the main optical path 414 by way of branchunit 430.

In the first embodiment of a branch unit, such as shown in FIG. 2, theprocessor 240 of the branch unit receives the output of the photodiodedetector 220, and makes a determination as to whether or not the servicepath is operating normally. If the processor determines that the servicepath is not operating normally, then the processor sends control signalsto at least one of the last few line units on the second branch path(protection path), to instruct those line units to increase their poweroutput levels to a normal power output state. This may requirecommunicating to the terminal, followed by the terminal communicating tothe line units. The sending of control signals from the processor to theupstream line units is typically performed via the same fiber opticlines used for the optical signal through modulation of the signal orthrough an additional channel wavelength. Alternatively, the processormay notify a network management system (NMS in FIGS. 3 and 4), whichprovides control over the entire fiber optic system. When a failure onthe protection path is determined, the last few line units on the secondbranch path are instructed to increase their power output levels, toprovide the backup signal at an increased power level on the secondbranch path to the combiner when the primary signal on the first branchpath is not received at the combiner due to some fault on the firstbranch path.

In the first embodiment of the branch unit, the protection and servicefibers are preferably split evenly between each pair of shallow waterlegs (e.g., 4/4 restoration and 4/4 service fibers for each 8/8 fiberleg), to ensure that there is always a communications path to land. Thereceiving end can be treated as a 1+1 (or unidirectional path switchedring) by the network protection system, with switchover provided when afiber failure is detected. That is, if no signal is received by thethird line terminating equipment 115 which is supposed to receive theoptical signal on the receive service line, then the system changes overto provide signals received by the fourth line terminating equipment 120to the system at the receive end.

FIG. 5 shows a branch unit 500 according to a second embodiment of theinvention, in which a switch, shown as a 1×2 switch 510, is utilized inthe branch unit 500. Under normal operation, the 1×2 switch 510 isoperative to provide the service signal, received at a first input portof the 1×2 switch 510, to the output port of the 1×2 switch 510, wherethe output port corresponds to the beginning of the main optical path.The 1×2 switch 510 is preferably an ultra-high reliability switch. Whena photodetector 515 detects no signal for at least a fixed period oftime, which indicates a problem (e.g., fiber cut) on the first branchpath, a processor 520 receives this information, and instructs the 1×2switch (via control line 525) to switch to provide the protectionsignal, received on the second input port of the 1×2 switch, to theoutput port of the 1×2 switch. In the second embodiment, there is noneed to have the last few line units of the second branch path set to azero or low-power output state under normal operating conditions, sincethere is no issue with respect to interference between signals receivedby the 1×2 switch on its two separate input ports. That is, in thesecond embodiment, all of the line units 108A, 108B on the first andsecond branch paths have their respective pump amplifiers in the lineunits always set to a normal power output state under normal operatingconditions.

For both the first and the second embodiments of a branch unit describedherein, failure of a service path detected by the photodiode is veryfast since there are few if any propagation delays, and thus theprocessor can be notified of a problem on a service path very fast andcommand a switch to a protection path. Reconfiguration times ofsubstantially less than a few milliseconds can be achieved from firstdetection of a failure on a service path, to switching to an appropriateprotection path in the second embodiment. In the first embodiment,reconfiguration times under 100 milliseconds can be achieved where extratime is needed for the line units in the protection path to boost theirpump amplifier power levels after being controlled to do so by eitherthe NMS or the branch unit directly. Thus, 1+1 and UPSR protection forthe receiving end at the landing station can be done transparently tothe branch switching by inserting a small delay, also a fewmilliseconds, which requires waiting after alarms in a channel for thatduration before channel level (or fiber level) switch over at thelanding station.

A fiber optical collapsed ring architecture has been described accordingto several embodiments of the present invention. Many modifications andvariations may be made to the techniques and structures described andillustrated herein without departing from the spirit and scope of theinvention. Accordingly, it should be understood that the methods andapparatus described herein are illustrative only and are not limitingupon the scope of the invention. For example, the description ofcomponents and units as given above may be utilized for eitherland-based units or for underwater units. The only difference is thatthe underwater units (e.g., repeaters, switches and branch units) aretypically hermetically sealed.

1. A fiber optic system, comprising: a primary transmission pathprovided from a source; a backup transmission path provided from thesource; a branch unit provided at a meeting point of the primary andbackup transmission paths, the branch unit including: a switch forreceiving primary and backup signals on the primary andbackuptransmission paths, respectively, and for outputting one of theprimary and backup signals onto an output port of the switch; aprocessor for receiving signal strength information of the signal on theprimary transmission path, wherein, in a first mode of operation, theswitch set to provide the primary signal to the output port of theswitch, wherein, in a second mode of operation, the switch is set toprovide the backup signal to the output port of the switch, wherein theprocessor instructs the switch to operate in one of the first mode ofoperation and the second mode of operation, based on the receivedinformation, a splitter provided at a second end of a main optical path,a first end of the main optical path corresponding to the output port ofthe switch, wherein, at the second end, the splitter splits a signaltraveling on the main optical path onto a first branch path that iscoupled to a destination, and a second branch path that is coupled tothe destination, and wherein the destination inputs signals from one ofthe first and second branch paths.
 2. The fiber optic system accordingto claim 1, further comprising a detector for detecting a signalstrength of an output signal on the output port of the switch, and forproviding information regarding the detected signal strength to theprocessor.
 3. The fiber optic system according to claim 1, wherein thefirst and second branches terminate at a single node.
 4. The fiber opticsystem according to claim 3, wherein the first and second branch pathseach terminate at a single node and connect through the primarytransmission path to another node, thereby providing point to pointconnection between the nodes with 1+1 redundancy in branched paths. 5.The fiber optic system according to claim 1, wherein the primary andbackup transmission paths provided from a first source each terminate ata single node.
 6. The fiber optic system according to claim 5, whereinthe primary and backup transmission paths each terminate at a first andsecond branch unit, and wherein the first and second branch paths foreach branch unit each terminate at a single node, thereby providing aconnection between at least four nodes.
 7. The fiber optic systemaccording to claim 1, wherein the fiber optic system is a submersiblefiber optic system.
 8. A method of providing fiber optic signals on afiber optical network, the method comprising: providing, from a source,primary signals on a primary transmission path; providing, from thesource, backup signals on a backup transmission path; receiving theprimary and backup signals on the primary and backup transmission paths,respectively, and outputting only one of the primary and backup signalsonto an output port that correspond to a main optical path; detecting asignal strength on the main optical path; determining, based on signalstrength or quality whether or not to operate in a first mode ofoperation, in which the primary signal is providing to the main opticalpath, or in second mode of operation, in which the backup signal isprovided to the main optical path; providing one of the primary signaland the backup signal on the main optical path; splitting the signal onthe main optical path into a first signal on a first branch path and asecond signal on a second branch path; and inputting to a destinationone of the first signal on the first branch oath and the second signalon the second branch path.
 9. The method of claim 8, wherein the fiberoptical network is a submersible fiber optical network.
 10. The methodof claim 8, wherein the first signal on the first branch path containsinformation identical to information in the second signal on the secondbranch path.
 11. The method of claim 10, wherein the first signal on thefirst branch path and the second signal on the second branch path areeach 3 dB less in signal strength than the signal on the main opticalpath which was split into the first and second signals on the first andsecond branch paths.